This invention relates generally to techniques and systems for measuring one or more parameters with optical fiber sensors.
During the mid to late 1970's, many suggestions and development efforts were directed toward measuring various parameters with a specially designed optical sensor provided at an end of a length of optical fiber, the other end of which is connected to a measuring electro-optical instrument. Much of this work was motivated by the desire to provide a non-metallic sensor that could be used in the presence of electromagnetic fields without either the measurement being affected by the fields or the sensor itself perturbing the fields.
U.S. Pat. No. 4,016,761 - Rozzell et al. (1977) describes the use of liquid crystal material as a temperature sensor, such a material having a varying absorptive characteristic as a function of temperature. U.S. Pat. No. 4,140,393 - Cetas (1979) proposes the use of birefringement material as a temperature probe. U.S. Pat. No. 4,136,566 - Christensen (1979) suggests the use of the temperature dependent light absorption characteristics of gallium arsenide for a temperature sensor. U.S. Pat. No. 4,179,927 - Saaski (1979) proposes a gaseous material having a temperature dependent light absorption.
U.S. Pat. No. 4,075,493 - Wickersheim (1978) suggests the use of a luminescent material as a temperature sensor, exciting radiation of one wavelength range being passed along the optical fiber from the measuring instrument, and temperature dependent luminescent radiation being emitted from the sensor back along the communicating optical fiber for detection and measurement by the instrument. It is the luminescent sensor technology which has found the greatest commercial applicability in fiber optic measurements, primarily for reasons of stability, wide temperature range, ability to minimize the effect of non-temperature light variations, small sensor size and the like. An example of a current commercial technology is given in U.S. Pat. No. 4,652,143 - Wickersheim et al. (1987).
Optical fiber temperature measurement techniques have been pursued for use primarily in applications where traditional electrical temperature sensors, such as thermistors and thermocouples, do not function well. One such application is in a strong radio frequency or microwave field, as previously mentioned. An example of this is a measurement of the temperature of an object being heated by a microwave field in an industrial heating, drying or curing application. Another example is the measurement of temperature of a human by implanting a sensor within an area of the body being heated by microwave energy, such as is used in the cancer treating hyperthermia techniques.
As the fiber optic temperature measurement technology has become commercially accepted, there has been a growing demand for similar devices that measure additional parameters, such as flow, pressure, index of refraction, or humidity. An example of a luminescent fiberoptic probe that can be used to measure the velocity of fluid flow, among other related parameters, is given in U.S. Pat. No. 4,621,929 - Phillips (1986). Infrared radiation is directed to the sensor along the optical fiber and is absorbed by a layer of material provided for that purpose. Once heated, the sensor is then allowed to be cooled by a flow of fluid, such cooling being measured by the luminescent sensor. The rate of cooling is proportional to the heat transfer characteristics and flow of the surrounding liquid.
U.S. Pat. No. 4,752,141 - Sun et al. (1988) describes a luminescent sensor for simultaneously measuring pressure and temperature. An elastomeric optical element is attached to an end of an optical fiber with luminescent material being coated on an outer convex surface of the optical element. Force supplied to compress the optical element deforms the luminescent coated surface and thus affects the luminescent optical signal in a way that can be used to detect the magnitude of such force independent of, and simultaneously with, temperature.
It is a primary object of the present invention to provide improvements in both single parameter and multi-parameter optical sensing technology.
It is also an object of the present invention to provide optical fiber sensors that are particularly useful in medical applications. In this connection, it is an object of the present invention to measure several useful parameters with a single sensor having as small a dimension as possible.